We all know that nearly every commercial game in existence has glitches, and that you can find and exploit them by simply looking them up online. But which ones did you find out on accident, without ever having had someone else tell you? I can think of a few off the top of my head:

The "die during the credits" glitch in SMG2

The glitch in Yoshi's Island where manually flutter jumping the first time gets you higher than letting the game do it automatically.

The "smashola" glitch in SMW.

The glitch in SMG where you can spin jump after jumping while sliding down an unclimbable slope.

there's a game called 'jump ultiamte stars' for the ds where i have found tons of glitches. it's a fighting game that i don't recommend anyone play ever-- it's awful and has a terrible community where ever you go.

besides that all i can think of is the glitch in the first taiko no tatsujin ds game where if you save 3 files it deletes everything (which was fixed). hm.. I know i've found more, but i just can't recall.

I haven't found many glitches in commercial games on my own, but I have found a bunch of typos, particularly in the Ace Attorney series. My favorite would have to be from Ace Attorney Investigations:

"He was placed under arrested."

I did find a glitch in Golden Sun: Dark Dawn, though. There was one area where you had to push a bunch of boxes around the room to solve a puzzle. If you pushed a box in a certain area, it'd create a one-tile wide gap between a rock and the box that the player could enter. If you went into that gap, then the player would become stuck in it and not be able to walk back out, plus you couldn't push the box back in the other direction. Luckily, there was a Retreat ability the character had that instantly teleported you back to the dungeon entrance, so it wasn't much of a problem.

In Drawn to Life: The Next Chapter I was surprised to find two glitchs.

1. In the space world, after the cutscene where the woman sneezes (just before the rocket ship level, or in my case, Rocket Cheese level) and the cop arrests her (by pushing her offscreen) they are supposed to be gone, but if you move toward where they went, you can still see them, although they are in a place where you can't reach, it kinda looks like the cop is raping her, heh.

2. In the final world, after you draw the first pillar, and collect the first orb, in the cutscene of the orb being placed on the pillar, it doesn't do anything, and everyone just stands there like an idiot, so I had to press start and skip the cutscene, bleh.

Here's a glitch yoshi discovered in Super Mario Galaxy 2. In the honeyhop galaxy, if you have mario land a certain way on the edges of the lily pads, mario will trip. What makes this a glitch is the fact that mario will keep tripping over and over again until you reset the game since there is no way to stop him (When this happened, the enemies were also too far away, so they couldn't help.) It was actually pretty funny when this happened.

Some would include:
-Bringing items through doors
-Making a Silver POW stay Silver even after passing a pipe, without any fixes in the hack
-Getting two items from a single item block by hitting the block with Mario's fist as well as an item at (nearly) the same time
-Mode7 and Yoshi/Items don't work well together
-Spawning multiple items simultaneously while a boo ceiling is present will cause one of the sprites to overwrite a boo, making it fall from the sky.

--------------------Of course you should fight fire with fire, you should fight everything with fire.

Probably the glitch in Rayman Raving Rabids where you save and re-start the game (On a specific in-game day) You spawn outside of your room in some white madness and just keeps falling. The wierdest thing is, that there is sound effects from wind and there is an falling animation.. Weird..

Anyway, you can't get away from there (No matter how hard you try) and at the second it starts you can see a glimpse of your room and some other stuff. Weird.

--------------------YouTube <-- Let's Plays and stuff. ^ Made by Tahixham

In Lego Harry Potter Wii, there's a glitch where if you switch characters while talking to the portrait of the fat lady, the portrait will NEVER open again.

Ah, that reminds me:

In the PC Harry Potter games (I played these to death as a kid since they were some of the only games I had) I discovered the following:

In HP1, collecting the first Wizard Card while jumping on Fred and George's heads will result in Harry facing the floor during the animation.

In HP2, casting Flipendo on one of the toilets in Moaning Mertle's bathroom can create an endless supply of gnomes.

In HP2, when you bounce off of a Spongify tile, the game allows you to fall from any height without sustaining fall damage (so that you don't die from jumping on the things). But if you climb onto a ledge in mid-air after bouncing off of one, the game will not reset the flag, giving you one free "survive any height" fall.

In HP2, when playing on Windows 98, jumping up a certain slope at a certain angle will allow you to jump off of a torch outside the castle, allowing you to go out of bounds.

Not so much a glitch, but in HP2 there's an unused level/spell challenge you can get to by activating ghost mode and going into one of the doors in the common room.

In HP3, through clever platforming you can climb up the branches of one of the trees in the courtyard, and from there you can jump over a wall and end up outside of the level's boundary.

Bowser's Castle on SMB3. On the alternate room to Bowser. Once you reach his lair and bringing a P-Wing, fly to the left until you find another room which happen to be Bowser's 2nd lair. You would then start fighting a glitched Bowser that doesn't breath fire. If done correctly, that is.

I honestly don't recall many glitches I found on my own. I guess the biggest would be the screen warp bug in Link's Awakening (you know .. the one where you press SELECT at the exact moment a screen transition is happening). I don't even remember -how- I found it, but yeah.

Aside from that .. nope, not much. I probably found a bug or two in YI since I overplayed that game when I was younger, but I can't recall specifics.

Oh yeah, speaking of Yoshi's Island, I did find two invalid screen exit issues on my own. The first being the one when trying to access the bonus room in 4-7. If you hug the left side of the wall while jumping in the passage to the bonus area, then you activate an unset screen exit, and end up falling from the skies of 1-1. Being the naive kid I was, I thought I found some super-secret area.

Same thing happens in the bonus room in 2-5. When you come out of the pipe into the area, if you quickly hover back up, you can re-enter the pipe and end up in the same area of 1-1.

The former issue was fixed in the GBA port, but surprisingly, the latter one was still present.

Speaking of Yoshi's Island, yoshi found another glitch. In 2-1, in the room with the part that scrolls up while stone blocks fall from above, if you enter the bonus room then return to the area with the stone blocks, the blocks will glitch up. If you jump down that pit, you will get stuck forever. Also in 2-1, if you ground pound poochy and hold it while poochy jumps against the left wall of the room, yoshi will pound the wall many times on the way up. Both of these glitches were fixed in the GBA version though.

I can't think of too many specific examples, but I have found quite a few on my own.

The game I remember breaking the most was Donkey Kong 64, where I found a ton of glitches where you could go through walls and do things out of order, and a lot of other useless things. Like, I remember figuring out how to get into Banana Fairy Island with any Kong. I'm sure everything I found was documented on sites somewhere eventually, but this was really before I started going online a lot.

I once found a glitch in NBA Jam (SNES version) that allows you to be "on fire" without scoring three baskets in a row by committing a goaltend, which for some reason doesn't count as a basket for the other team though they are awarded the amount of points the goaltended shot was worth.

A long time ago, I discovered a glitch in Mario & Luigi: Superstar Saga on my own-- if you shrink Mario with Luigi's hammer and go to a new area where the music is supposed to change (the green warp pipes work well for this), the music of the old area may remain (a few places like Joke's End will still load their normal music though). The incorrect music even stays after battles. (A year or two later though, I found it was already known online (and had been since before I discovered it myself). But it was nice to be able to discover something.)

Also, in Donkey Kong 64, I accidentally glitched through a wall at least once on my first playthrough. That game is ridiculously full of wall clipping glitches. (By the way, for anyone wondering where they're documented, try Speed Demos Archive.)